1. Field of the Invention
The present invention relates to a catalytic converter, and more particularly to a catalytic converter for use in an internal combustion engine exhaust system.
2. Description of the Related Art
A principal objective of catalytic converters is to regulate exhaust gases emitted from a vehicle without significantly increasing the energy consumption of the engine. A catalytic converter generally includes a catalytic substrate having a honeycomb structure with a plurality of small holes that are arranged systematically with respect to the flow direction of exhaust gas. Honeycomb substrates tend to minimize pressure losses and to improve the efficiency of the catalytic converter.
The temperature of the exhaust gas varies according to the engine operative conditions. More specifically, the exhaust gas is held at low temperature while starting and idling the engine. The temperature of the exhaust gas increases until it reaches normal operational temperatures. The method for efficiently purifying the exhaust gas, even at its maximum temperature, by employing catalytic converters, to meet the engine full load heat capacity, is known. Immediately after the engine is started, the exhaust gas throughput is small, and its temperature low.
Consequently, a catalytic reaction does not commence, since the catalyst temperature is not sufficiently high to cause undesirable elements, such as formaldehyde, to be treated and filtered by the catalatic converter. As a result, the exhaust gas becomes highly toxic at lower temperature. An attempt to improve the efficiency of the exhaust gas purification process, at low to normal temperatures, has been made, by installing several different heat capacity catalysts.
Japanese Unexamined Utility Model Publication Hei 2-19818 discloses a catalytic converter of this type. The catalytic converter has a substrate shown in FIG. 9, and two metal catalysts 100 and 101 of different capacities. These catalysts 100 and 101 are disposed within a tube 102 along the direction of flow of the exhaust gas emitted by the internal combustion engine. The catalyst 100 has a smaller capacity, and is designed to purify lower temperature exhaust gases. The catalyst 101 has a larger capacity and is designed to purify higher temperature exhaust gases. However, if the coefficient of thermal expansion of the catalysts 100 and 101 were greater than that of the tube 102, thermal stress could develop, and might cause damage to the catalysts 100 and 102. One attempted solution has been to increase the contractile strength of the catalysts 100 and 101, even though this will increase the weight of these catalysts. As a result, the installation of the catalytic converter in a vehicle becomes more difficult.